1. Field of the Invention
The present invention relates to a method for making a packaging material. More particularly, the invention relates to a method for making a selectively metallized material and package blank for use in microwave heating packages.
2. Description of the Prior Art
The heating of food articles in microwave ovens is now common. Because the molecular friction resulting from microwaves heats materials without browning them or making them crisp in the manner customary with certain foods (e.g., pizza, waffles, fish sticks), various methods of obtaining browning and crisping have been tried. One method that is now achieving wider acceptance is the use of disposable packaging including one or more microwave interactive bodies positioned in contact with or adjacent to the food surfaces or portions desired to be browned and/or crisped, e.g., U.S. Pat. Nos. 4,230,924, 4,267,420, 4,555,605, and 4,590,349.
The packaging material disclosed in U.S. Pat. No. 4,267,420 represents an early effort to make a package material that allows microwave browning of foods by including a thin layer of a microwave interactive substance, such as aluminum, in the packaging material. In U.S. Pat. No. 4,267,420, aluminum is evaporatively deposited onto a plastic film, such as a polyester film. The resulting metallized film is laminated to an additional sheet or film made preferably of polyethylene (although more rigid dielectric materials such as paperboard and the like are also suggested), so that the aluminum microwave interactive layer is sandwiched between two plastic films. This produces a flexible packaging material that can be used as a wrap for fish sticks or other items requiring surface browning. U.S. Pat. No. 4,230,924 discloses an earlier version of this packaging material in which the metallic layer is subdivided into a number of rectangular metallic islands or pads having sides ranging from 1/32 inch to 3/4 inch. Also shown in that patent is a plastic film or paperboard substrate that has the metallic islands on one side, namely the side intended for contact with the food to be browned. No second layer of material is contemplated to form a sandwich with the metallic layer in the center.
Another form of packaging material, one that has been put into commercial use, is a composite material in which a metallized polyester film is laminated onto a lightweight kraft paper. This kraft paper is then cut into pieces of the desired size and patched onto pre-die cut paperboard carton blanks. While this places the microwave interactive material in the desired position, the patching operation is an inefficient manufacturing process. Moreover, while the paperboard carton portions covered by the metallized polyester film and kraft paper are protected from moisture and grease, unless the remaining paperboard has been covered with a protective layer, it is vulnerable to these substances. On the other hand, the presence of such a protective layer over the entire paperboard may make it more difficult to effectively attach the patch of microwave interactive material to the paperboard.
U.S. Pat. Nos. 4,230,924 and 4,267,420 are concerned mainly with defining and describing the configuration of the layered packaging materials involved. Details on the manufacture of the materials, in particular on the mass production of such packaging materials, are sparse. Efficient mass production is necessary if inexpensive disposable packages are to achieve wide use. The deficiencies of the commercially used package described in the preceding paragraph demonstrate that there are practical problems of mass production of microwave heating packages that need to be addressed.
While the preparation of a composite packaging material with microwave interactive qualities appear straightforward, on closer examination, the microwave oven cooking environment poses a number of special problems. First, in microwave cooking, browning may be desired only in certain portions of a food item, such as the edges or the fried potatoes in a multiple food item tray ("TV dinner"). Thus, only portions of a package require a microwave interactive layer. Second, microwave ovens have limited power. This makes it desirable to avoid placement of microwave interactive material where no food is contacted, because heating in such areas is primarily wasted. Third, the same qualities that permit metallized materials to heat up and brown an adjacent food surface can lead to arcing between the metallized material and the oven floor or between overlapped package layers, each of which includes the metallized material (e.g., at a glue joint). All of these factors suggest the desirability of selectively placing microwave interactive materials only where needed on the packaging material. The patch application method previously mentioned can do this but is not suited for efficient mass production.
Another requirement of the microwave cooking environment is that adhesives, coatings and other materials used in packaging must not emit significant amounts of chemicals that may be harmful or that may cause undesirable effects on color, smell or taste of the cooked food products. A further requirement in the microwave cooking environment is that packaging materials that are used for heating foods by intimate contact must not adhere to the food products unduly. These latter factors make it necessary to carefully select all materials in a composite packaging material, particularly the outer layer that contacts food.
Further, a composite material, if constructed of layers, must be joined by adhesives that provide adequate bonding. The bonds must survive freezing or refrigeration as well as microwave oven heating. A package that falls apart before cooking would be unsaleable or at least unappealing to consumers. A package that falls apart during cooking can adversely affect the packaged food. In addition, if the adhesive is to adhere to a selectively metallized plastic film, it must provide adequate bonding both for the metallized and non-metallized portions, which may have quite different surface characteristics.
Additional requirements for microwave heating package materials arise due to the economics of packaging. If microwave cooking packages are to be disposable, the package blanks must be made from quite inexpensive materials. Moreover, the package blanks should, if possible, be suitable for processing on conventional package-making and filling machinery.
What is lacking in the prior art is a method for efficiently producing inexpensive packaging materials and packaging blanks in which microwave interactive layers can be selectively placed where desired.